The present invention relates to electrical circuits for comparing the values of two input signals, and more particularly to comparators whose outputs do not exhibit fluctuating output levels in the presence of noise in the input signals.
"Comparators" are circuit elements whose function is to compare the values of two input signals. Although the term is most often used to refer to comparators which compare the amplitudes of two analog input signals, the term is broad enough to encompass also frequency comparators, phase comparators, current comparators, etc. Other types of comparators include those which function in the digital domain rather than in the analog domain, comparing values of two digital input signals. Comparators of these various kinds are basic circuit elements finding extensive use in a large variety of divergent applications.
Although the comparators specifically disclosed herein are of the variety which compare the amplitudes of two analog input signals, it will be appreciated that the techniques and circuits disclosed herein can readily be applied to other forms of comparators, both digital and analog; the invention is therefore not restricted to the specific types of comparators with which it is described in reference to.
Comparators, in general, provide bilevel output signals indicating the relative values of the two input signals supplied thereto. When one of the input signals has a value which is greater than the value of the other input signals, the output has the first value. When the value of the first input signal is less than the value of the second input signal, the output signal has a second value. In many circumstances it is necessary or desirable that the comparator provide unambiguous switching between the first and second output levels as the values of the two input signals approach and cross over. If some noise is associated with one or both of the input signals, however, such noise may cause the relative values of the two input signals to cross over a number of times in rapid succession, producing multiple output transitions where only a single one is desired.
One method utilized in the past to avoid the multiple transitions in the output signals is to provide the comparator with some degree of hysteresis. A hysteresis comparator avoids noise transitions in the output by having different triggering points for positive and negative transitions in the output of the comparator. The two trigger points are selected to be far enough apart that noise in one or both of the input signals does not cause excursions in the relative value of the two signals across both trigger points.
Comparators with hysteresis are not "precise", in the sense that they do not trigger when the values of the two input signals become equal. Instead, there is a small "dead" band centered about the equal relative values wherein on triggering takes place. The existence of this dead band, included to prevent noise triggering of the comparator, is undesirable in those circumstances in which the comparator is intended to provide a precise indication of the relative values of the two input signals.